This invention relates to polarization prisms, and, more particularly, to polarization prisms that are constructed from birefringent or biaxial materials.
In the prior art, there are basically four techniques for polarizing a natural beam of light. These techniques employ either Brewster's angle plates, a pair of prisms with or without an intermediate layer which may be constructed from a birefringent or biaxial material, dichroic materials such as "polaroid," and wire grids or conductive gratings.
The Brewsters's angle plate polarizers require the use of a stack of closely ground and polished transparent plates canted at the complement of Brewster's angle to the direction of travel of the incident light. Polarizers of this type may be constructed for use in the transmission mode, reflection mode, or both. In the transmission mode, light of the unwanted polarization is removed from the beam by preferential reflection at the canted surfaces, while the desired polarization is transmitted. In the reflection mode, the reflected beam of light is taken as the desired output, while the transmitted light is regarded as unwanted and disposed of by absorption or other means. It is also possible to use this type of polarizer as a beamsplitting polarizer, the reflected light being predominantly of one polarization and the transmitted light being predominantly of the other. There are difficulties associated with each mode of operation of this type of polarizer, all of which stem from the fact that only for light incident on a surface at precisely Brewster's angle is the reflected beam entirely of one polarization and that even then only 20-30% of this one polarization is reflected with the balance being transmitted. In the transmission mode the 70-80% of the above-mentioned polarization which is transmitted is undesired, and many surfaces must typically be cascaded in order to obtain a device with usably high rejection of the undesired polarization. This requirement for a multiplicity of surfaces increases the size and cost of such a polarizer and creates additional problems in the ultraviolet portion of the spectrum, where few materials are entirely without absorption and requirements on quality of surface finish in order to avoid light loss and depolarization due to scatter are more severe than they are at longer wavelengths. In the reflection mode, the lack of purity of the polarization of the reflected beam for angles of incidence other than precisely Brewster's angle severely restricts the usable angular aperture of such a device. Furthermore, since only 20-30% of the desired polarization is reflected by a single surface, the throughput of such a polarizer is quite low unless many surfaces are cascaded, as in the case of transmission mode operation. In addition to the difficulties created by such a multiplicity of surfaces in the transmission mode case, the further difficulty then arises that the spaces between the surfaces (i.e., the thickness of the plates and the spaces between them) must be very small in order that the output beam not be distorted by elongation or "smearing."
The second polarization technique utilizes birefringent material in one of several prism types. One type utilizes a polarization prism that polarizes the incident light by total internal reflection of one of the two electric field components of the incident light at an interior surface which is canted to the incident light at or beyond a selected critical angle. A second type utilizes a polarization prism which transmits both electric field components of the incident light physically separating them from each other at the output of the polarization prism.
Several techniques have been utilized in the prior art to construct a polarization prism using the first of the above birefringent prism types. One widely used technique for implementing this type of polarization prism is to cut one or more calcite crystals to form a Nicol or a Glan Thompson type prism. The resultant prism parts are then "cemented" together with Canada balsam, oil, or other cement with an appropriate index of refraction. Another implementation of the calcite polarizer is to "cement" a layer of calcite between two glass prisms. The use of a calcite polarizer for broadband applications is undesirable because calcite is fragile, generally difficult to polish, is very expensive, and does not readily transmit ultraviolet light in the 2000A wavelength region. In addition, the "cements" used in the construction of these, and similar polarizers of the prior art, tend to discolor and dry out with age, and do not readily transmit ultraviolet light.
Other techniques of producing the first type of birefringent polarization prisms include trying to grow a birefringent crystal between two prism faces, replacement of the non-birefringent prisms with liquids of a selected refractive index, and by grinding and polishing the mating surfaces of the prisms and a central layer of a birefringent material so that the surfaces match to an accuracy of within 1/10 of a wavelength of the shortest wavelength light to be polarized, thus requiring no cement to assemble these polarization prisms. The technique of growing the birefringent crystal between the two prisms presents problems in maintaining the proper orientation of the crystalline optic axis during growth of the crystal, and in the minimization of the residual strain in the crystal after cooling. The effect of the residual strain is a reduction in the crossout (i.e., percent polarization of the incident light), thus limiting the optical quality of the polarization prism. Typical problems with the liquid prisms are the need for ancillary devices for the liquid, the fluctuation of the refraction of the liquid with age as a result of evaporation and other characteristic changes, and the lack of the transmissibility of ultraviolet light through these liquids. Also, while the technique of closely grinding and polishing the mating surfaces of the materials is a viable solution of many of the problems introduced by the cement techniques for visible light, it is not for ultraviolet light. The wavelength of ultraviolet light is on the order of one half to one quarter that of visible light wavelengths, thus, the technique of closely grinding and polishing the mating prism surfaces would necessitate grinding and polishing those surfaces to a tolerance which is one half to one quarter the necessary tolerance for visible light. These smaller tolerances greatly increase the cost of the prism and may be unachievable with relatively soft prism materials.
The second type of birefringent polarization prisms are the Wollaston and Rochon shearing type polarizers. These polarizers produce two plane polarized, orthogonal light beams with an angular separation between them at the same output surface of the polarization prism. Thus, it is necessary to use a mechanical obstruction to eliminate one of the two light beams. In addition, the Wollaston polarizer disperses both polarizations of the incident light, and the Rochon polarizer yields only one half the angular separation of the polarized light beams of the Wollaston polarizer. Further, these shearing type polarizers are also subject to the same material limitations as the Glan Thompson and Nichol type prisms. Therefore, the usefulness of this type of polarizer is limited in many applications.
The third polarization technique typically utilizes a thin layer of tiny needlelike dichoric crystals of herapathite, in parallel orientation, embedded in a plastic matrix and enclosed for protection between two transparent plates. Polarization with this technique is limited to narrow band applications, provides incomplete polarization, and has a high transmission loss.
The phenomenon of the fourth polarization technique utilizes a conductive grating or wire grid, where the spacing between the conductive elements is less than the wavelength of the highest frequency light component of the incident light beam. This technique has been used for a number of years and is more fully described in U.S. Pat. No. 3,536,373 and in the paper Bird, G. R. and Parrish, M., Jr., "The Wire Grid as a Near-Infrared Polarizer," Journal of the Optical Society of America, Vol. 50, No. 9, pp. 886-891, Sept. 1960. Wire grid polarizers have been used successfully into the infrared region, but their use at higher frequencies is limited by the fineness of the required wires and the small spacing between those wires which cannot be produced with the present state of the art.